(a) I or (b) |
CH.C0H
CHAP, xv LACTIC ACID
107
Now lactic acid can be got from aldehyde, CH. { . CHO, by
adding to it HCN, and boiling the product with hydrochloric
acid (see pp. 64 and 69) :
CH 3 . CHO
Aldehyde. Lactic acid.
and we are thus led to assign to lactic acid the formula (t>) of
the two given above.
The lactic acid in sour milk is produced from the lactose or
milk - sugar present in milk by the action of a particular
ferment. Cane-sugar, starch, and other carbohydrates also
yield lactic acid under the influence of the same ferment :
C i2 H 22n + H 2 = 4C 3 H O 3 .
Milk or cane sugar. Lactic acid.
In preparing lactic acid the following is a good method of
procedure :
One kilogram of cane-sugar is dissolved along with about 5 grams of
tartaric acid in 3^ litres of water ; after a few days some rotten cheese
(30 grams) is rubbed into a paste with sour milk (i^ litres), and added
to the solution with 400 grams of zinc oxide. The whole is left to
ferment in a warm place for a week or ten days. Then the mixture is
heated to boiling, filtered, and the filtrate evaporated. Crystals of zinc
lactate separate out on cooling ; they are collected and dissolved in water,
and the zinc removed by passing H 2 S. The zinc sulphide is removed by
filtration, and the solution of lactic acid evaporated on the water bath.
Lactic acid thus obtained is a thick syrupy liquid. The sodium
salt has the formula C 3 H 5 NaO 3 , but when this is heated with
metallic sodium, a second atom of the metal is introduced in
place of the alcoholic hydrogen, and a substance of the
formula C 3 H 4 Na 2 O 3 is obtained.
Lactic acid can also be prepared by several synthetical methods :
1 i ) From aldehyde CH 3 . CHO (see above).
(2) From the bromopropionic acid, CHy . CH 2 Br . CO^H, and potash.
Great interest attaches to the existence of an isomeric para-
lactic acid, which is present in the juice of meat. This
io8 CITRIC ACID
behaves exactly like ordinary lactic acid in nearly every other
respect, but differs from it in being able to rotate .the plane of
polarisation of light. This is connected by Van't Hoff, with
the fact that one carbon atom in lactic acid is " asymmetric,"
that is, connected to four dissimilar radicles. For a fuller
account of this theory, see the second part of this book.
There is also known another acid, hydracrylic, which is isomeric
with lactic acid. The formula (a) given above (p. 106) is indicated for it
by its formation from ethylene as indicated below :
CH 2 CH 2 OH CH 2 OH CH 2 OH
CH 2 CH 2 C1 CH 2 .CN CH 2 .CO. 2 H.
(+HC1O). (Action of KCN). (Boiling with HC1).
Citric Acid is found in lemons, currants, cranberries, and
many other sour fruits. It is prepared commercially from lemon
or lime-juice by means of the calcium salt.
Its formula is found by analysis to be C 6 H g O 7 , and it
behaves as a tribasic acid. It contains, therefore, three
carboxyl groups, and forms salts, such as C 6 H 6 O 7 K 3 , and
ethereal salts, such as C ( .H 5 O 7 (C 2 H 5 ) T In these the action of
acetyl chloride proves the existence of an hydroxyl group (see
p. 79). The acid therefore contains one OH and three CO 2 H
group, and its formula may be written C 3 H 4 (OH)(CO 2 H) y
Citric acid crystallises in large prisms. As a tribasic acid
it forms three series of salts, the three potassium salts being
C 6 H 7 O 7 K, C 6 H 6 O 7 K 2 , and C 6 H 6 O 7 K 3 .
Calcium citrate, (C 6 H 5 O 7 ) 2 Ca 3 , is remarkable as being less
soluble in hot water than in cold, a property made use of in
testing for citric acid.
EXPT. 19. To some solution of citric acid in a test tube add lime
water until the reaction is slightly alkaline. No precipitate is formed in
the cold, but a white precipitate of calcium citrate appears on boiling.
QUESTIONS ON CHAPTER XV
i. How can lactic acid be obtained from sugar? Why is its formula
written C 3 H 6 O 3 and not CH 2 O ?
CITRIC ACID
109
2. Mention some other substances which have the same percentage
composition as lactic acid. How could you distinguish them ?
3. What happens when (a) milk turns sour, (b) butter turns rancid, (c)
wine goes sour ?
4. Write down the formulae of (a) the three potassium citrates, (6) zinc
lactate.
CHAPTER XVI
THE ALLYL COMPOUNDS
THE allyl compounds may be regarded as being derived from
the hydrocarbon propylene, C 3 H 6 , and their starting-point
allyl alcohol stands to propylene in the same relation as
ethyl alcohol does to ethane.
FIG. 30. Preparation of Allyl Alcohol.
Propylene has the formula CH : CH . CH 3 , and from this
three alcohols might be derived :
1. CH(OH) : CH . CH 3 , a secondary alcohol,
2. CH 2 : C(OH) . CH :J> , a tertiary ,,
3. CH 2 : CH . CH 2 (OH), a primary
ALLYL ALCOHOL
Of these, the third formula represents ally! alcohol, which
in many respects behaves like any other primary alcohol,
but differs from methyl alcohol and its homologues in being
unsaturated (see p. 28). On the one hand, as a primary
alcohol, it yields an aldehyde and then an acid when oxidised,
while as an unsaturated compound it is able to combine
directly with chlorine or bromine.
Allyl Alcohol, C 8 H 5 . OH, is obtained by distilling a
mixture of glycerine, C. t H 5 (OH) s , with formic acid (oxalic acid
may be substituted for this, but as it decomposes under these
conditions into formic acid and CO 2 , the reaction is practically
the same). The formic acid is oxidised to CO 2 and water :
CjH^OH^ + HCO,H = C :j H 5 . OH + CO 2 +2H 2 O.
Glycerine. Formic acid. Allyl alcohol.
The following is the usual method for preparing allyl
alcohol :
Four parts of glycerine and one of crystallised oxalic acid are placed in
a retort and gradually heated. At first much CO 2 is evolved, and dilute
formic acid distils over. When the temperature of the mixture reaches
190 the receiver is changed, and impure allyl alcohol is obtained as the
distillate. This is purified by fractional distillation, and freed from water
by treatment with anhydrous baryta. Pure allyl alcohol boils at 96.
Allyl alcohol is a colourless liquid which, like all the allyl
compounds, has an irritating, unpleasant smell. As an un-
saturated body it combines directly with Cl., or Br 2 to form
derivatives of propyl alcohol :
C 3 H 5 . OH + Br 2 = C 8 H 5 Br . OH.
Allyl alcohol. Dibromo-propyl
alcohol.
As a primary alcohol allyl alcohol yields, when carefully
oxidised, first an aldehyde allyl aldehyde or acrolein and
then an acid acrylic acid :
CH 2 : CH . CH 2 OH + O = CH 2 : CH . CHO + H 2 O
Allyl alcohol. Acrolein.
CH, : CH . CHO + O = CH 2 : CH . CO,H.
Acrolein. Acrylic acid.
ALLYL IODIDE
CHO, is also produced when glycerine or
fats (which are compounds of glycerine) are heated to decom-
position. It is best obtained by distilling glycerine to which
twice its weight of KHSO 4 has been added :
C 3 H (OH) 3 = C 2 H 3 . CHO + 2H 2 0.
Glycerine. Acrolein.
Acrolein is a volatile liquid with an extremely irritating
odour. Its chemical behaviour is fairly summed up in the
statement that it is an unsaturated aldehyde.
Acrylic Acid, C 9 H 3 . CO 9 H, is best obtained from acrolein
by boiling it with water and oxide of silver :
C 2 H 3 . CHO + Ag.,O = C 2 H 3 . CO 2 H + 2 Ag.
Acrolein. Acrylic acid.
Acrylic is a well-marked acid. It is of course an un-
saturated body and, as such, combines readily with chlorine,
bromine, etc., to form derivatives of propionic acid :
C 2 H 3 . CO 2 H + Br 2 = C 2 H 3 Br 2 . CO 2 H.
Acrylic acid. Dibrom-propionic acid.
It is a liquid similar to acetic acid in appearance and smell.
Allyl alcohol forms ethereal salts with acids, but of these
the following are alone of sufficient importance to be mentioned
here :
Allyl Iodide, C 3 H 5 I, is a colourless liquid, which can be
obtained from the alcohol by the action of H I :
C 3 H 5 .OH + HI = C 3 H 5 I + H 2 0,
Allyl alcohol. Allyl iodide.
or more conveniently from glycerine by the action of phos-
phorus and iodine, a reaction which may be supposed to occur
in the two following stages :
C 3 H 6 (OH), + PI 3 = C 3 H 5 I 3 + H 3 P0 3
Glycerine. Glyceryl iodide.
C 3 H 5 I 3 = C 3 H 5 I + I 2
Glyceryl iodide. Allyl iodide.
OIL OF GARLIC
The experimental details of the second method of prepara-
tion are as follows :
A quantity of glycerine is freed from water by heating in an open dish
for at least half an hour to such a temperature that the liquid is near its
boiling point and evolves abundant fumes. The anhydrous glycerine must
be placed in a well-stoppered bottle while still warm.
A tubulated retort is fitted with a cork and connected with an
apparatus for generating CO 2 , so that a slow current of that gas can be
passed through the retort during the whole experiment ; 150 grams of the
anhydrous glycerine is then placed in the retort, along with 100 grams of
powdered iodine ; 60 grams of yellow phosphorus is weighed out and cut
into small pieces, which are taken up one by one at the end of a knife,
dried between filter-paper, and introduced through the tubulus into the
retort. A violent reaction occurs as each piece of phosphorus is added,
and impure allyl iodide distils over ; it is washed with a solution of soda,
separated by means of a tap-funnel from the soda, dried by contact with
a few pieces of fused calcium chloride, and re-distilled. Pure allyl iodide
boils at 101 C.
FIG. 31. Preparation of Allyl Iodide.
Allyl Sulphide, (C 3 H 5 ) 2 S, is the chief constituent of oil of
garlic, which is obtained by distilling garlic with steam, ami
gives that plant its characteristic smell and taste. It can be
prepared artificially by the action of allyl iodide upon
potassium sulphide :
K 2 S
== 2KI
Allyl iodide.
(C 3 H 5 ) 2 S.
Allyl sulphide.
Allyl Iso-thiocyanate, C..H 5 . NCS, is present in oil of
I
ii4 OIL OF MUSTARD
mustard, obtained by distillation of mustard seeds. It can be
prepared artificially by the action of allyl iodide upon potassium
thiocyanate KCNS :
KCNS + C 3 H 6 I = KI + C 3 H 5 . NCS.
Oil of mustard.
It is a liquid with the strong penetrating odour and taste of
the natural "oil of mustard."
QUESTIONS ON CHAPTER XVI
1. How can allyl alcohol he obtained from glycerine? What reactions
stamp allyl alcohol as an unsaturated compound ?
2. By what reactions is it possible to prepare acrylic acid from
glycerine ?
3. What reasons have we for regarding allyl alcohol as an unsaturated
primary alcohol ?
4. Give the formulae and systematic names cf (a) oil of mustard, (6)
oil of garlic. How can each be prepared artificially ?
CHAPTER XVII
GLYCERINE AND ITS COMPOUNDS
Glycerine is contained in fats and fatty oils combined
with organic acids in the form of ethereal salts. When these
compounds are heated with alkalies in the preparation of soap
the glycerine is set free, and when the soap is separated by
addition of salt from the liquor in which the glycerine is
contained, this latter can be easily recovered. In many soaps
now manufactured the water and glycerine are not separated
from the true soap, but the whole is allowed to cool, when it
solidifies to a mass naturally less firm than a pure soap and
less durable, but pleasanter to use and far more profitable to
manufacture. Soap manufacture is accordingly not a very
important source of glycerine ; far more is obtained in the
preparation of stearic acid for candles. The best process
conducts the saponification of the fat by means of superheated
steam with the use of a small proportion of lime. Stearic
acid (mixed with other fatty acids) and glycerine are
produced :
Fat + Water = Stearic Acid + Glycerine.
Glycerine is found by analysis to have the formula C 3 H g O r
It behaves as a trihydric alcohol, and yields ethereal salts
with various acids, in which three acid groups are introduced
into the glycerine molecule ; this leads us to write the formula
as C 3 H 5 (OH) 3 .
Glycerine when perfectly pure forms colourless crystals
which melt at 17 C, about the ordinary temperature of a
room. It is, however, very hygroscopic, and a trace of water
u6 GLYCERINE
is sufficient to convert it into a syrupy liquid ; this has a
sweet taste, and is sometimes added to wine to give it body
and sweetness. It is also used as a cosmetic and for keeping
leather articles soft and pliable ; it is the starting-point in the
manufacture of nitro-glycerine and dynamite.
When distilled under the ordinary pressure, glycerine is
largely decomposed, acrolein being one of the principal
products :
C 3 H 5 (OH) 3 = C 3 H 4 + 2H 2 0,
Glycerine. Acrolein.
but under diminished pressure or in a current of superheated
steam it can be distilled without decomposition.
Glycerine can be prepared synthetically from allyl tribromide
CH 2 Br . CHBr . CH 2 Br, just as glycol from CH 2 Br . CH 2 Br and ethyl
alcohol from CH 5 Br ; its constitutional formula is"CH 2 (OH) . CH(OH) .
CH 2 (OH), and when oxidised it yields first glyceric and then tartronic
acids :
CH 2 . OH
I
CH . OH - >
I
CH 2 . OH
The most important compound of glycerine is the nitrate,
generally known as nitro-glycerine ; this is obtained by the
action upon glycerine of a mixture of concentrated sulphuric
and nitric acids ; the product is added to water when the
nitro-glycerine separates as an oil, which has to be thoroughly
washed before being stored or worked up into dynamite, as
otherwise the traces of acid left in the oil render it liable to
explode on very slight provocation.
Nitro-glycerine has the constitution C 3 H 5 (NO 3 ) ;J ; it is the
nitrate of the tri-valent radicle C 3 H 5 (glyceryl), and its
formation is represented by the equation :
C 8 H 6 (OH) 3 + 3 HN0 3 = C 3 H 5 (N0 8 ) 3 + 3 H 2 O.
Glycerine. Glyceryl nitrate
or nitro-glycerine.
The sulphuric acid used in its manufacture merely aids the
NITRO-GLYCERINE 117
action of the nitric acid by combining with the water
produced.
By boiling with water and an alkali, nitro-glycerine (like
other ethereal salts) is converted into the alcohol and acid
from which it was formed :
C 3 H 5 (N0 3 ) 3 + 3 KOH . C 3 H 5 (OH) ;J + 3 KNO 3 .
Nitro-glycerine. Glycerine.
Nitro-glycerine, like most other similar compounds (see gun-
cotton, p. 126), decomposes very readily when heated or
exposed to sudden shock. The substance contains more
oxygen than is required to burn up the carbon and hydrogen
contained in it :
hence no oxygen from outside is required, and nitro-glycerine
can burn or explode when cut off from contact with air.
Moreover, the oxygen with which the carbon and hydrogen
combine is present in the same molecule with them, and in
consequence the change represented in the above equation
takes place with extreme rapidity and suddenness when once
started. The heat produced in the reaction is therefore also
very suddenly developed, and the destructive power of nitro-
glycerine is far in excess of that of a quantity of gunpowder,
which in burning would give out the same total amount of
heat.
Nitro-glycerine is a very dangerous substance to handle, as
even when very carefully prepared it requires only a slight
shock to make it explode. This disadvantage is largely
removed in dynamite, which is a mixture of nitro-glycerine
with very fine siliceous earth. More recently this has been
almost superseded by blasting - gelatine, a jelly-like solid
obtained by dissolving gun-cotton in nitro-glycerine, which is
even safer to handle, and can, by varying the proportions, be
made in different grades of violence according to the purpose
intended. By addition of camphor, or other appropriate
substance to this mixture, a material is obtained of sufficiently
moderate explosive power to be used in ordinary firearms
the modern smokeless powder.
n8 THE CHLORHYDRINS CHAP, xvn
Of some theoretical interest are the chlorhydrins,
compounds obtained from glycerine by the action of HC1 or
of PC1 5 ; in these the hydroxyl groups of the C.jH 5 (OH) 3 are
more or less completely replaced by chlorine ; they are
ethereal salts of the trihydric alcohol glycerine and hydro-
chloric acid.
There are two mono-chlorhydrins. (a) CH,(OH) . CH(OH). CH 2 C1
and (/3) CH 2 (OH) . CHC1 . CH 2 (OH), of which the first is obtained by
the action of HC1 on glycerine.
Of the two di -chlorhydrins one has the formula CH L ,C1 . CH(OH) .
CH..C1, and is obtained by the action of HC1 on glycerine ; the other one
is CH 2 C1. CHC1. CH 2 (OH). and is the addition product of allyl alcohol
(see p. in) and CU
Trichlorhydrin, C 3 H 5 C1 3 (CH 2 C1 . CHC1 . CH 2 C1), is the
final result of the action of HC1 (or better, PC1 5 ) upon
glycerine ; it is one of the five possible isomeric trichloro-
propanes, and is a liquid with a smell like chloroform.
QUESTIONS ON CHAPTER XVII
1. What is the chemical constitution of fat ? How are the fats worked
up in the manufacture of glycerine ?
2. What happens to glycerine (a) when heated in the air, (b) when
treated with a mixture of nitric and sulphuric acids?
3. What chemical changes occur when nitro-glycerine (a) explodes,
(b) is heated gently with dilute caustic soda ?
4. How is the dangerous violence of nitro-glycerine modified in several
modern explosives?
CHAPTER XVIII
THE CARBOHYDRATES
The Carbohydrates are a class of bodies of extreme
importance, especially in plant life ; not only are they the
chief constituents of all plants, but they are also present in
many animal tissues.
All the carbohydrates are composed of the three elements,
carbon, hydrogen, and oxygen, and of these elements the two
latter are present in the proportion in which they combine to
form water ; their number is very large, and their accurate
investigation is surrounded with such difficulties that only
in recent years has much real knowledge of their chemistry
been gained.
The chief difficulty was that no reagent was known with which the
carbohydrates would yield well-characterised products ; the compounds
which they, as aldehyde and ketone-alcohols, form with phenyl-hydrazine
can, however, for the most part be distinctly and easily recognised, and it
is by their help that much of our recent knowledge in this field has been
won.
THE GLUCOSES
The first family of the carbohydrates to be considered is
the Glucoses ; these have the empirical formula CH 2 O, and
most of them have the molecular formula C 6 H 12 O 6 , as has
been proved by the application of Raoult's method for the
determination of molecular weights (see p. 16).
There are, however, bodies known of the molecular formulae C 5 Hj O 5
(arabinose) and C 7 H 14 O 7 (heptose), which are best included in this group.
THE GLUCOSES
The glucoses have a sweet taste, though less sweet than
cane-sugar ; they are easily soluble in water, and at once
reduce Fehling's solution (p. 62) ; they also readily ferment
under the influence of yeast. The various glucoses differ
from one another in crystalline form, in their solubility in
various reagents, and in other properties ; their isomerism
cannot be satisfactorily accounted for by the ordinary theories
of the structure of carbon compounds, and its fuller explana-
tion is undoubtedly to be found in the application of Van't
HofPs theory of the tetrahedral carbon atom (see p. 108).
In view of this, special importance is attached to the varying
power of the different glucoses to rotate the plane of polarisa-
tion of light.
Chemically the glucoses are, in the first place, alcohols ;
they (at least those of the formula C 6 H 12 O 6 ) contain five OH
groups (each of which can be replaced by acetyl upon treat-
ment with acetic anhydride, see p. 79). In the second place,
the reducing power of the glucoses leads to the conclusion that
they are aldehydes or ketones as well as alcohols ; they contain
therefore five OH groups and one CHO or CO group.
This CHO or CO group can be converted by reduction into
a sixth alcohol group CH 2 OH or CHOH. We thus obtain
by reduction of a glucose a hexhydric alcohol, which may be
regarded as the parent of that particular glucose. The alcohol
obtained has in each case the formula C 6 H 8 (OH) 6 , but while
two of the glucoses (dextrose and levulose) yield mannitol, a
third (galactose) yields an isomer of that substance, viz. duldtol.
Mannitol is also contained in manna, and is present in many plants,
as is also the isomeric duldtol. Both are derivatives of normal hexane,
C 6 H ]4 , and their isomerism is to be explained by Van't Hoff's theory of
the tetrahedral carbon atom.
Dextrose, C^H^Og, is present in many fruits, and also in
honey. It rotates the plane of polarisation to the right.
Dextrose is formed in the hydrolysis (splitting up of com-
pounds by addition of water) of many other carbohydrates. The
hydrolysis is effected by heating with water under pressure, or
more easily by boiling with a dilute mineral acid. Thus we
have the following reactions :
Cane-sugar + H 2 O = Dextrose + Levulose
Starch + H 2 O = Dextrose
DEXTROSE AND LEVULOSE
The dextrose of commerce is prepared by treating starch with boiling
dilute sulphuric acid under pressure. The solution is freed from sulphuric
acid by adding calcium carbonate and filtering from the calcium sulphate ;
it is then evaporated, and leaves a tough non-crystalline mass.
Levulose, C,.H 10 O 6 , occurs along with dextrose in fruits
and honey, and the " invert-sugar " obtained by the action of
dilute acids on cane-sugar is a mixture of equal parts of dex-
trose and levulose.
Levulose rotates the plane of polarisation more strongly
than dextrose, but to the left.
The dextrose and levulose which are present together in honey and in
invert-sugar can be partially separated by washing the mixture with cold
alcohol. The more soluble levulose is thus removed dissolved in the
alcohol, and the less soluble dextrose remains for the most part un-
dissolved.
Galactose, C 6 H 10 O 6 , is formed along with dextrose in the
hydrolysis of milk-sugar :
Milk-sugar + H O = Dextrose + Galactose.
Unlike dextrose and levulose, galactose does not ferment with
yeast. When reduced it yields dulcitol :
Rotation of
polarised light.
Action of yeast.
Reduction
product.
Dextrose
To right
Ferments
Mannitol
Levulose
To left
Ferments less
rapidly than
glucose
Mannitol
Galactose
To right
Does not fer-
ment
Dulcitol
CANE-SUGAR GROUP OR BIOSES
The members of this group are made up of two molecules
of glucose united together with elimination of a molecule of
HYDROLYSIS OF SUGAR
water. When hydrolysed (see above) they take up water to
form two molecules of glucose. The formula is C^H.^O,,,
and the following table indicates the relation of the most im-
portant members of the group to the glucoses :
Cane-sugar + H 9 O = Dextrose + Levulose
Milk-sugar + H 2 O = Dextrose + Galactose
Maltose + H 2 O = Dextrose + Dextrose
The Bioses are not so strong reducing agents as the Glucoses.
None of them is able to reduce Fehling's solution in the cold,
FIG. 32. Sugar-cane.
Yield of canes per acre, 30-40 tons,
containing about 5 tons of sugar.
FIG. 33. Sugar-beet.
Yield of beet per acre, 15-20 tons,
containing about 2 tons of sugar.
but maltose does so readily when heat is applied,
reduce it only very slowly even when boiled.
The others
CANE-SUGAR
123
Cane-sugar, C 12 H 29 O n , is present in the sap of many
plants, especially the sugar-cane and the beet-root. In order
to obtain the sugar the sap is extracted either by crushing
and pressure, or by cutting into thin slices and soaking in
water. The juice is purified by filtration and other processes,
and is then evaporated in vacuum-pans until sugar separates
out from the juice on cooling.
A portion only of the sugar is thus obtained in the crystal-
line state, the remainder is left in the form of a thick syrup
after the crystals have been removed, and the sugar in it is
prevented by impurities from crystallising. These " molasses "
may either be fermented and converted into spirit (rum), or
by certain modern processes the impurities may be separated
and the sugar obtained in the solid form.
In one of these, the diffusion process, the syrup is put into
what are practically huge bags made of parchment paper, and
these bags are placed in pure water. The sugar of the molasses
diffuses through the pores of the parchment paper faster than
the impurities which are mixed with it, and there is thus ob-
tained in the liquor surrounding the bags a solution of sugar